In some cases, it can happen that crashlogs don't have any signal in
the exception, which causes the parser to crash.
This fixes the parsing by checking if the `signal` field is in the
`exception` dictionary before trying to access it.
rdar://84552251
Differential Revision: https://reviews.llvm.org/D119504
Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>